There has been known an organic electroluminescence device (hereinafter, referred to as an “organic EL device”) that includes an emitting unit (in which an emitting layer is included) between an anode and a cathode and emits light using exciton energy generated by a recombination of holes and electrons that have been injected into the emitting layer.
Recently, an arrangement of the organic EL device having a plurality of the emitting units laminated via an intermediate unit and connected in series has been studied for improving luminescent efficiency and lifetime of the organic EL device. Such a device arrangement is occasionally called a tandem-type.
The tandem-type organic EL device requires holes and electrons to be supplied from the intermediate unit to the adjacent emitting units. Accordingly, the intermediate unit is required to have a function for generating the holes and the electrons as well as donating the generated holes and electrons to each of the adjacent emitting units.
From such a background, various studies regarding the intermediate unit have been made.
For instance, an intermediate unit of an organic EL device disclosed in Patent Literature 1 (JP-A-11-329748) is provided by inserting a transparent conductive film such as ITO and IZO between an electron injecting layer and a hole injecting layer, and generates and injects charges to the respective emitting units.
Moreover, for instance, the intermediate unit disclosed in Patent Literature 2 (JP-A-2006-66380) is provided by laminating an electron pulling layer for pulling electrons out of an adjacent layer adjacent to the cathode and the electron injecting layer on the electron pulling layer near the anode. First and second emitting units are adjacent to the intermediate unit. A hexaazatriphenylene derivative is used for the electron pulling layer. Alkali metals (e.g., Li and Cs), alkali metal oxides (e.g., Li2O) and the like are used for the electron injecting layer. The intermediate unit supplies holes, which are generated by the electron pulling layer pulling the electrons out of the adjacent layer, to the first emitting unit and supplies the pulled electrons to the second emitting unit via the electron injecting layer. Thus, driving voltage of the organic EL device is reduced.
For instance, a charge generating layer of the organic EL device disclosed in Patent Literature 3 (JP-A-2006-173550) is provided by laminating an interface layer and an intrinsic charge generating layer. An oxide (e.g., Li2SiO3) containing at least one of an alkali metal and an alkali earth metal is used for the interface layer (intermediate cathode layer). A hexaazatriphenylene derivative and the like which can provide charge injecting efficiency at the same level as V2O5 are used for the intrinsic charge generating layer. The charge generating layer is stabilized by providing the interface layer and the charge injecting layer in the emitting unit is omitted by providing the intrinsic charge generating layer, which simplifies the layer arrangement.
In addition to the arrangement of the intermediate unit, as disclosed in Patent Literatures 2 and 3, provided by laminating a layer formed of an alkali metal, an alkali metal compound or the like and a layer formed of an organic compound such as the hexaazatriphenylene derivative, an arrangement of the intermediate unit provided by laminating inorganic compounds has been considered.
For instance, an intermediate layer as the intermediate unit of the organic EL device disclosed in Patent Literature 4 (JP-A-2010-92741) is provided by laminating a conductive layer and a hole injecting layer, or by laminating a conductive layer and an electron injecting layer. For the conductive layer, a mixture of ITO and SiO and the like in addition to Au, ZnO, IZO (indium-zinc oxide) and the like are used as an electric conductor having a low specific resistance (1×105 Ω·cm or less). For the hole injecting layer, a metal oxide capable of injecting holes (e.g., WO3, MoO3 and a mixture of ZnO and Mo) is used. For the electron injecting layer, a metal oxide capable of injecting electrons (e.g., Li2O and Na2O) is used. Thus, driving voltage of the organic EL device is reduced by providing the conductive layer having a low specific resistance. A charge injection barrier to the emitting layer adjacent to the intermediate layer is lowered by providing the hole injecting layer or the electron injecting layer reduces, whereby the driving voltage is further reduced.
Patent Literature 5 (JP-A-2004-281371) discloses a linking unit provided by laminating an n-type doped organic layer, interface layer and p-type doped organic layer, in which the interface layer is lead oxide.
Additionally, Patent Literature 6 (JP Patent No. 3933591) discloses an organic EL device including a charge generating layer provided by laminating an electron accepting substance and an electron donating substance as an intermediate unit. A charge transfer complex is formed on an interface between the electron accepting substance and the electron donating substance. When voltage is applied, charge is moved to the emitting unit adjacent to the charge generating layer, which realizes a long lifetime device in a high luminance range.
However, when the intermediate unit disclosed in Patent Literature 1 is employed as the intermediate unit of the tandem-type organic electroluminescence device, transparent conductive films of ITO and IZO are formed by a sputtering process and electron-beam evaporation. However, in the sputtering process, it is considered that a neutral particle and a charged particle of high speed (tens of electron volts (eV) or more) which are derived from generated plasma cause damages to an organic substance, thereby deteriorating the electron injecting layer of the device.
When the intermediate units disclosed in Patent Literatures 2 and 3 are employed, the layer formed of an alkali metal or an alkali metal compound and the layer formed of the hexaazatriphenylene derivative are likely to form a highly resistant layer. Consequently, the device has to be driven at a high voltage and luminescent efficiency thereof is reduced.
Also when the intermediate layer is provided by laminating the inorganic compounds as disclosed in Patent Literature 4, the device has to be driven at a high voltage and luminescent efficiency thereof is reduced.
In Patent Literature 5 in which lead oxide is used as the interface layer, since lead oxide is an oxide having a low melting point, satisfactory drive durability could not be obtained.
When the charge generating layer disclosed in Patent Literature 6 is employed as the intermediate unit, visible light is absorbed by the charge transfer complex, resulting in reduction in luminescent efficiency.